If you have just installed a refilled, remanufactured or compatible ink cartridges it uses a chip to identify itself to the printer. These chips use a combination of counting pages and average page coverage to estimate the amount of ink inside the cartridge. As they have no way to check the physical ink level in a cartridge, a chip that has been used previously will almost certainly warn of having low ink or even being empty once installed. As your printer will not warn you when the ink is running low just keeping an eye on the quality of your prints and replacing the relevant cartridge when a colour begins to fade.